Ensure that LN channel can be updated during split channel update

[luckysori]

We have no tests to verify that we can handle receiving updates to the commitment transaction via LDK whilst we are in the middle of an LN-DLC protocol (open/close sub-channel).

LDK will need to update the LN commitment transaction automatically to route payments or when updating fees. A consumer of rust-dlc could avoid creating invoices or sending payments at certain points in time to prevent clashes, but the automatic updates are unavoidable.

I would recommend to write the tests first to reproduce scenarios in which we break the channel if these things happen simultaneously. And then we can proceed with implementing a solution.

Mentioned in this thread #116 (comment)